1. Field of the Invention
The invention relates generally to laser tracking systems. In particular, it relates to a laser tracking system which can locate a target in three or five dimensions.
2. Background Art
The use of robots in manufacturing has been rapidly growing for over a decade. Commonly, robots are used for purposes of improving quality and increasing productivity. However, robots are being applied to situations where the robot design and the associated control systems are insufficietly advanced for satisfactory performance. Many robots in present use were first designed for simple operations such as pick and place or spot-welding where accuracy and kinematic performance were not crucial. However, newer robot applications, such as assembly, deburring, seam-welding, inspection, machining, drilling and the like require better performance and accuracy than that originally required of earlier robot systems. The prospective use of such systems must be able to specify robots in terms of the parameters essential for verifying that the procured robot meets these specifications before it is installed. Because of these factors, there is a growing world-wide effort to develop internationally accepted procedures and terminology to measure and describe robot performance.
One of the most difficult problems is that of determining the positional accuracy of an industrial robot throughout its large work zone. Just the fact that a robot has been instructed to assume some position does not guarantee that it has in fact done so. Several positional problems exist. The required accuracy many exceed the accuracy of the motive power. Although Pryor in U.S. Pat. No. 4,453,085 describes a robot positioning detector in rectilinear coordinates, positions in most robots are typically controlled by encoders attached to rotatable joints. The encoders may not be adequately measuring the positional change. In the extended work zone, there may be extreme positions in which a small variation in the motive power or the encoders results in large positional variations. A moving robot arm and its attached load may have large inertia. As a result, dynamic or kinematic effects may cause variations over otherwise correct static positions. Robot loads are variable and robot members have some unavoidable elasticity. Therefore, positions may be load dependent.
Although many techniques have been developed for an assessement of the accuracy of measuring machines and machine tools, these techniques are often unsuited to robots because of the less stringent accuracy requirements and a smaller work zone of the prior techniques. A particular difficulty is caused by the absence of linear axes on most robots.
The generalized problem faced in robot metrology is that of tracking a point in three-dimensional space and measuring its location. One way to accomplish this task is by attaching a reflector, hereafter called a target, to the measurement point and tracking its location optically. Several versions of the scheme have been described, such as by the present inventors in a technical article entitled "A Survey of Current Robot Metrology Methods", appearing in the Annals of the CIRP, Vol. 33, No. 2, 1984 (CIRP stands for International Conference of Production Research), or by Gilby et al. in a technical article entitled "Laser Tracking System to Measure Robot Arm Performance" appearing in Sensor Review, October 1982, pp. 180-184.
These techniques include stereo-triangulation with theodolites or photogrammetry, multiple length measurement with laser, acoustics or wires and multiple camera-like systems. Stereo-triangulation is undesirable since it requires a minimum of two tracking systems and it is a static measuring technique. Similarly, imaging by camera is undesirable since the resolution of the system is too low for robot positioning measurements.
A further positioning difficulty, somewhat unique to robots, is than in sophisticated robot arms, not only must the end of the robot arm be positioned in three-dimensional space but the tool or the like attached to the end of the arm has two or three additional degrees of rotational freedom about the end of the robot arm, namely pitch, yaw and roll. That is, the tool holder on the end of the arm must be accurately positioned in at least five degrees of freedom. The sixth degree of freedom, roll, will not be directly discussed here. Both stereo-triangulation and camera systems become relatively complicated for the extra two degrees of freedom.